Radio communication devices often use filters for a wide variety of applications. In one such example, a bandpass filter is used in determining the selectivity of a radio receiver. Bandpass filters vary in structure and composition. Most bandpass filters include reactively coupled resonators, whose resonant characteristics define the filter's center frequency of operation and selectivity response. Generally, the resonators are coupled to each other by reactive components such as capacitors, inductors, or combinations thereof. A number of factors, including the type and number of resonators in the filter help determine the selectivity of a filter.
Bandpass filters may have fixed or tunable pass band center frequencies. Filters having fixed center frequencies are usually used in narrow band receivers with the operational frequencies limited to a narrow radio frequency (RF) bandwidth. On the other hand, filters having a tunable pass band frequency responses may be used in wide band receivers where the range of the operational frequency covers a substantially large bandwidth. Tunable filters are particularly suitable in reducing manufacturing costs of radio communication devices as a single circuit can be used to cover a wide frequency band, thus eliminating the need for multiple designs covering only portions of a frequency band.
A filter may be tuned by adjusting the value of selected reactive elements within the filter's topology. Generally, reactive values are changed through electronic components whose reactive characteristics are dependent upon voltage or current sources. For example, variable reactance can be accomplished through electrical components whose capacitive characteristics are dependent upon a bias voltage. Many tunable filters are varactor based. A resonator within the filter may include a tuning element having one or more varactors which respond to a control voltage to provide tuning for the filter. By varying the control voltage, the filter may be tuned to provide a particular pass band center frequency for a desired frequency of operation. A typical prior art tunable filter includes two resonators, coupled by back-to-back varactors, such that upon simultaneous application of a control voltage, the center frequencies of the resonators are varied. Accordingly, the tuning elements receive identical control voltages, and as these control voltages vary, the filter is tuned for a desired frequency response.
In designing a tunable filter, many tradeoffs must be made between costs, complexity, and performance characteristics. Moreover, adjustments in portions of the design to improve one performance characteristic, may have a negative effect on another performance characteristic.
A bandpass filter (BPF) typically consists of multiple resonator structures that are reactively coupled together to achieve the desired RF filter response. A BPF provides a low insertion loss for signals within a particular pass band frequency range, and rejects signals that are outside the pass band. One known implementation uses a capacitive element as a coupler between pairs of inductive resonator structures. The coupling factor between the resonator structures is determined by the capacitive value of the coupler and is indicative of the loading of the resonator structure. A high series capacitor value results in a low series reactive impedance, which increases the loading effects of the inductive resonator structures. Conversely, a low series capacitive value results in a high series reactive impedance which provides for less coupling, and consequently less loading of the resonator structures. Varying the coupling by utilizing varactor diodes is typical in the art. To mitigate the non-linear response of a varactor, a filter is ordinarily designed to utilize only the linear range of the varactor response. This can require additional circuitry to generate higher control voltages to provide adequate tuning.
It is desirable to have a filter, such as the bandpass filter, that provides a linear response to tuning. Prior art filter designs utilizing low cost components, such as varactors provide limited tuning capabilities without additional costly circuitry. Therefore, a new approach to the design of tunable filters is required.